Fluid flow control valves are commonly employed in diagnostic instruments to regulate fluids through the device. Operation of the flow control valve is important to ensure proper operation of the diagnostic system. This is especially true in systems having multiple fluids that must be selected quickly during operation. Rotary shear valves are one type of fluid flow control valve and have long been a standard design for flow control used in precision flow control applications.
A conventional rotary shear valve includes a generally disc-shaped rotor in contact with a disc-shaped stator. A typical rotary shear valve comprises a stator having one or more ports (e.g., through holes) and a rotor having one or more slots. For example, the stator may include a common port, a sample port, one or more fluid ports, and a waste port. The rotor rotates with respect to the stator to selectively fluidly connect ports of the stator via a slot on the rotor. Exemplary rotary shear valves are disclosed in U.S. Pat. No. 3,567,389 to Coulter et al.; U.S. Pat. No. 3,567,390 to Rothermel; and U.S. Pat. No. 4,507,977 to Cabrera.
In operation, the rotary shear valve may be mounted to a fluid manifold and the rotor may rotate to establish a fluid pathway through the rotary shear valve comprising select stator ports and a rotor slot. The fluid pathway may connect a fluid source to a fluid mixing chamber on the manifold, for example. When the rotor rotates to close off the pathway, the supply side of the stator will have an unmixable volume of fluid. The volume of the unmixable fluid is dependent on the size of the stator port. For example, where the stator ports are circular through holes, the volume of unmixable fluid will be substantially equal to πr2h, where r=radius of circular face of the port, and h=height or thickness of the stator. This unmixable volume of fluid is undesirable.
One way of reducing the unmixable volume of fluid is to reduce the thickness of the stator. A problem with reducing the thickness of the stator, however, is that a thinner stator has a tendency to deform when clamped or forced against a valve housing surface. Deformation of the stator causes the valve to leak, which is also undesirable. For this reason, conventional stators tend to be relatively thick (e.g., greater than 3 mm). As described above, a thicker stator results in an undesirable larger volume of unmixable fluid.
Deformation of the stator in a rotary shear valve due to a clamping force may result from the following condition. One conventional design approach is to have a clamping force on both sides (e.g., top and bottom) and around the entire periphery of the stator. For example, a stator seating force on the top side of the stator may be provided by an o-ring that acts to seat a bottom surface of the stator against a relatively flat valve housing rim. In a typical design, the stator is made from a ceramic material and includes a highly polished flat surface, and the valve housing rim is made of metal. Although attempts are made to ensure an equal and opposite force around the entire periphery of the stator, the fact is that the metal surface of the valve housing rim is not completely flat, at least not when compared to the highly polished flat surface of the ceramic stator. As such, the stator seating o-ring force around the entire top periphery of the stator seats the stator against the three highest points of the valve housing rim and the stator deforms like a potato chip. This results in undefined and random contact mounting deformation between the stator and rotor. This condition may lead to leaking of the rotary shear valve.
Another problem with some conventional rotary shear valves is that they include a valve housing having an upper and bottom stator seating rim. These designs require a large, heavy spring to properly seat the stator and to provide adequate sealing force between the rotor and the stator. Large, heavy springs also typically require use of a relatively thick stator. In addition, a large, heavy spring also results in higher torque requirements to rotate the rotor, which requires a larger, more powerful motor. This may also lead to a reduction in the valve switching speed, which is also undesirable.
In other conventional designs, the stator may be top loaded. For example, a compression force may be asserted on top of the stator to seat the bottom of the stator on the valve body rim. In such conventional rotary shear valves, the stator is permanently affixed to the structure to which it is mounted (e.g., a fluid manifold). For example, an epoxy is used to permanently affix and seal the stator to the manifold. This is undesirable since any failure of the valve requires replacement of both the rotary shear valve and the manifold. Also, over time the epoxy will degrade and lose its bond causing delamination of the epoxy and valve failure.